1. Field of the Invention
This invention relates to movie film and video production, and more particularly to methods of and apparatus for processing a video signal in such production.
2. Description of the Prior Art
In the special effects now commonly used for making films for cinema or television viewing, it is often required to merge recorded live action with material which has been recorded using models. By the use of such special effects, it is possible, for example, to create the illusion of interaction between a live human actor and a two-meter white rabbit. This is achieved by filming the actor and the rabbit separately, the rabbit being a small model which is filmed using a single frame animation. The two resulting films are then merged into a single film using a matting technique. This procedure, and in particular the matting technique, is capable of substantial extension. In, for example, a science fiction film, a substantial number of independent models, which may for example represent a fleet of spacecraft, may be used, with a separate stage of matting for each model.
All these procedures, although they are often complex, time-consuming and expensive to carry out, are well known and are in common use.
There is, however, a significant problem with these procedures; which is that the final film often does not look correct because of differences in the movement characteristics of the live action material and the single frame animation material. Thus the animated material often has movement characteristics which are different from those of the live action material, and in consequence they look unnatural.
This problem arises because the live action material will have been recorded either with a movie film camera which has a finite shutter speed, typically 1/48 second for a standard movie camera with a 180.degree. shutter, or a video camera which has an effective shutter speed of one video field period, which is generally 1/50 or 1/60 second. In consequence any movement occurring while the shutter is open is integrated on the film or in the video signal, and the result is that on reproduction the image of any object that is in motion in the picture sequence is slightly blurred. When viewed, this slightly blurred image results in a natural characteristic for the movement. The effect may be considered as a temporal pre-filtering of the image prior to sampling.
In single frame animation, there is no such integration of the image. This is because although the movie film camera or video camera used has a finite shutter speed, the model does not move during the period that the shutter is open.
There have been some proposals aimed at overcoming this problem. One is the so-called "Go-Motion" technique which involves moving the model during the period that the shutter of the camera is open. In this technique the model is moved by devices such as small hydraulic motors, which are of course kept hidden from the camera. Such techniques are very time-consuming and expensive, not only because of the additional equipment which is required and the substantial labour involved in operating it, but also due to the complexity resulting from the need to plot the trajectories of all movements in detail before exposing each frame.